This invention relates to the formation of concrete pipe of uniform density and compaction.
In the packer head method of forming concrete pipe, concrete mix is fed by means of a motor powered conveyor to a cylindrical mold located beneath the conveyor containing a rotating packer head for packing concrete against the inner wall of the mold. The packer head contains a plurality of peripheral rollers which act to compress the added concrete mix against the mold inner wall. The packer head is rotated by an electric motor coupled to a shaft centrally mounted on the packer head. A lifting device is also coupled to the shaft and motor to raise and lower the packer head within the mold.
The concrete pipe is formed by rotating and raising the packer head through the mold while concrete is fed by the conveyor to the mold. Reinforced concrete pipe containing a wire cage as well as unreinforced pipe can be made by this method.
Usually this is done in two stages called passes, each pass consisting of the raising of the packer head upwardly through the mold while concrete is fed thereto. The first pass serves to roughly form the pipe and the second pass performs a finishing operation on the inside surface of the roughly formed pipe by smoothing out internal surfaces and evening out the pipe inner diameter along its entire length. Control is carried out manually by an operator. Pipe made by this method is generally non-uniform in density and compaction. Such non-uniform density is a result of inefficient operator control of the apparatus. In the operation the operator visually assesses the uniformity of density of concrete and manually adjusts the rate of feeding concrete from the conveyor into the area of formation between the packing head and the mold as the article is being formed. Such operator-controlled procedures are highly inefficient and ineffective.
Non-uniform density of concrete in areas of formation as the article is being formed in such devices resulting from inefficiency of such operator-controlled procedures generates formation of stresses in the article. In the case of forming non-reinforced concrete articles by this method, such stresses generate cracks which substantially weaken the formed pipe resulting in damage and leakage during use. In the case of reinforced concrete pipe, such stresses generate bending and deflection of the reinforcing wire cage forming voids which also substantially weakens the pipe, resulting in damage and leakage during use.
In U.S. Pat. No. 3,746,494 to F. Gauger, a method for forming concrete pipe or tubes of uniform compaction is provided employing a packer head apparatus wherein the torque supplied by the packer head motor to the mold during pipe formation and which varies with the rate of supply of concrete is detected, the degree of compaction of the concrete being a function of the torque supplied by the motor, the rate of rise of the packer head is decreased when there is detected a decrease in the amount of torque and increased when there is detected an increase in torque. This uniform compaction and density of concrete pipe is formed according to this patent by varying the rate of packer head rise while the rate of concrete feed to the mold is held essentially constant.
The apparatus also includes a driven impeller mounted atop the packer head which is driven at a greater rotational speed than the rotational speed of the packer head and in a direction opposite to that of the packer head. The packer head is lifted by a hydraulic ram connected with the packer head shaft and connected by a pressure oil supply pump by means of a duct which is connected with a valve. This valve is responsive to the supply pressure of a hydraulic motor and connects the ram to a greater or lesser extent with an oil container as the pressure in the hydraulic motor supply duct varies.
The present invention, on the other hand, provides a method and apparatus for producing concrete pipe of uniform density by the packer head operation and compaction which employs a variable concrete feed rate but a constant rate of rise of the packer head as well as a constant packer head motor speed. The invention includes an automated system which measures horsepower of the packer head motor and through a feedback signal to the conveyor motor maintains a feed rate which will result in a constant horsepower and thus produce uniform density pipe. The apparatus of the invention may be computerized to coordinate all mechanical functions.